1. Field of the Invention
The present invention relates to twisted-pair cables used, for instance, in LAN (local area network) systems. The use of twisted-pair cables in a LAN system is principally motivated by their economical advantage in comparison to other types of cables. Moreover, the twisted-pair cables have the additional advantage of being easy to handle.
2. Description of Related Art
A twisted-pair cable 50, shown e.g. in FIG. 1, is manufactured by the successive steps of: coating a conductor portion 51 with an insulator coating 52, thereby forming an insulated core wire 53; twisting two of the insulated core wires 53 to produce a twisted-pair core wire 54; assembling four twisted-pair core wires 54 to produce a core cable assembly 55; and finally coating the core cable assembly 55 with a cable coating 56.
Recently, the transmission speed in LAN systems has increased tremendously. Taking this progress into account, a transmission speed of 250 MHZ has now been prescribed, for example, in the specification defined in Category 6 of EIA (Electronic Industries Association) and TIA (Telecommunications Industry Association) in the United States.
Consequently, the twisted-pair cables 50 used in LAN systems must also satisfy the requirements for such high transmission characteristics. In particular, voice (or conversation) leakage (or crosstalk) at a high-transmission speed must be lowered to minimum.
In the twisted-pair cable 50 shown in FIG. 1, the four units of twisted-pair core wire 54 are further referred to as {circle around (1)}, {circle around (2)}, {circle around (3)} and {circle around (4)}, respectively. When such a twisted-pair cable 50 is considered, six combinations of twisted-pair core wires 54 are conceivable as a cause for voice or conversation leakage. These combinations are {circle around (1)}/{circle around (2)}, {circle around (1)}/{circle around (3)}, {circle around (1)}/{circle around (4)}, {circle around (2)}/{circle around (3)}, {circle around (2)}/{circle around (4)} and {circle around (3)}/{circle around (4)}. Among these combinations, each of four combinations: {circle around (1)}/{circle around (2)}, {circle around (1)}/{circle around (4)}, {circle around (2)}/{circle around (3)} and {circle around (3)}/{circle around (4)} includes two twisted-pair core wires 54 which are adjacent to each other over their length and assembled in the circumferential direction in the twisted-pair cable 50. In these combinations, the two twisted-pair core wires 54 are constantly in contact over their length. They therefore tend to generate voice leakage, and cause deterioration in the sound quality of the conversation in the twisted-pair cable 50.
Even in the other combinations, i.e., {circle around (1)}/{circle around (3)}, and {circle around (2)}/{circle around (4)}, the cross-section of a twisted-pair core wire 54 does not form a proper circle, as such a twisted-pair core wire 54 is formed by twisting the insulated core wires 53. Accordingly, the shape shown in FIG. 1 tends to be distorted. As a result, the twisted-pair core wires 54 may be occasionally brought closer to each other, or even put into contact, in certain portions along the length of the cable 50. In such portions, the twisted-pair core wires 54 may be subjected to states alternating between contact and separation.
In order to diminish voice leakage, attempts have been made to coat each twisted-pair core wire 54 with an insulator layer or, as described in patent document JP-A-11-53958, to interpose a spacer having a cross-shaped section between pair of twisted-pair core wires 54.
However, when such known measures are relied upon, the insulator coatings or the use of spacers increase material costs and call for more process steps. Manufacturing costs of the twisted-pair cables are thus inevitably increased.
Moreover, when these coatings and spacers are added into the twisted-pair cables, the cables become thicker and stiffer, and their handling becomes more awkward.
Further, when cable ends are to be conditioned, the coatings and spacers must be removed beforehand. The end-conditioning process thus becomes less efficient.
Thus, some objective of the present invention are to solve such problems, and to provide a low-cost twisted-pair cable which reduces voice leakage, facilitates the handling process and eases the task of cable end conditioning process.
To this end, there is provided a twisted-pair cable including a pair of twisted-pair core wires respectively including a pair of insulated core wires, each of which includes an electrical conductor and an insulator layer coated thereon. According to one aspect of the present invention, the twisted-pair cable includes a core cable assembly formed by twisting a pair of unitary core wire complexes, each of which is formed by twisting the pair of twisted-pair core wires.
Preferably, the pair of twisted-pair core wires has a respective twist pitch. The twist pitches for each of the twisted-pair core wires contained in the same unitary core wire complex are then arranged such as to yield a unit turn number difference of at least 30 turns/m.
Further, the twist pitches for each of the twisted-pair core wires contained in different unitary core wire complexes may also be arranged such as to yield a unit turn number difference of at least 15 turns/m.
Preferably yet, the pair of unitary core wire complexes has a respective twist pitch, and the twist pitches for each of the unitary core wire complexes are arranged such as to yield a unit turn number difference of at least 15 turns/m.
According to another aspect of the invention, a method of making a twisted-pair cable is provided that includes twisting two pair of insulated core wires to form first and second pairs of twisted-core wires, twisting the first and second pairs of twisted-core wires to form a core wire complex, and twisting a pair of the core wire complexes to form a core cable assembly.
In another aspect of the present invention, the method of making a twisted-pair cable may also include performing the twisting of the two pair of wires by twisting the first pair of twisted core wires at a first twist pitch and twisting the second pair of twisted pitch wires at a second pitch different from the first twist pitch.
Other aspects of the method of making a twisted-pair cable of the present invention include configuring the first and second pitches to result in a unit turn of the respective twist pitches for the twisted-pair core wires contained in different core wire complexes to yield a unit turn number difference of at least 15 turns/m; or providing each of the core wire complexes with a respective twist pitch, and configuring both of the respective twist pitches for the core wire complexes to yield a unit turn number difference of at least 15 turns/m.
In still another aspect of the present invention, the method of making a twisted-pair cable may further include surrounding the core cable assembly with an outer insulating layer.